1. Field of the Invention
The present invention relates to a gateway unit, a control method thereof, and a communication system that perform real-time facsimile communication via a PSTN (public switched telephone network) and a packet network.
2. Description of the Related Art
In facsimile communication through a PSTN, specifically G3 (group 3) facsimile communication based on the ITU-T recommendation T.30, for example, a transmitting terminal and a receiving terminal transmits and receives, respectively, a facsimile message while directly exchanging a facsimile control signal. Since it is a real-time communication, a capability exchange between the transmitting terminal and the receiving terminal is possible. Thereby, when a reception error arises at the receiving terminal during communication, the error can be detected by exchanging the facsimile control signal between the transmitting terminal and the receiving terminal. Thus, the transmitting terminal can recognize the reception error as a transmission error by detecting a failure in the exchange of the facsimile control signal. Thereby, an advantage is that checking whether the facsimile message has been correctly transmitted to the receiving terminal is facilitated.
The facsimile communication through the PSTN, however, has a problem in that the communication fee is charged according to transmission time.
On the other hand, in packet networks, such as the Internet and the like, communication by E-mail is widely used. The network communication by E-mail has an advantage in that the communication charge is basically free.
In the network communication by E-mail, a mail server intervenes between the transmitting terminal and the receiving terminal. Therefore, transmission of the E-mail from the transmitting terminal to the mail server and reception of the E-mail from the mail server by the receiving terminal are performed on a non-real-time basis, that is, the two events do not take place continuously one after the other. Thus, the capability exchange between the transmitting terminal and the receiving terminal is impossible. Thus, if a reception error occurs during the reception from the mail server, even if the transmission of the E-mail has already been successfully completed at the transmitting terminal, it cannot be detected by the transmitting terminal.
Therefore, undesirable events have occurred when the receiving terminal was not prepared, for example, for a form (file format, coding form, resolution, and the like) of document data transmitted by the transmitting terminal. That is, although the transmission of the document data from the transmitting terminal was successful, a reception error occurred at the receiving terminal that is incapable of handling the data, resulting in a transmission failure.
Then, ITU-T recommendation T.38 was issued in April 1999, which is for exchanging a packetized facsimile control signal on a packet network. The recommendation provides a communication mode that offers advantages of both the PSTN and the packet network, namely, the capability exchange between the terminals and the real-time communication, being the advantages of the facsimile communication through the PSTN, and the basically free communication charge, being the advantage of the packet network.
By performing a communication based on the T.38 recommendation, a facsimile communication between the terminals, securing the capability exchange and in real-time, can now be performed on the network.
There are two types of network terminals that follow the T.38 recommendation (T.38 terminals), one being an IAF (Internet Aware Fax) that is directly connected to the network, and the other being a GW (Gateway) that performs a real-time transfer to a leased line, a PSTN circuit and the like.
The IAF type terminal is a so-called network facsimile apparatus, which exchanges a T.30 signal that is a packetized facsimile control signal with a partner terminal unit (IAF or GW) on a real-time basis, and serves as a final destination to receive document data from a partner terminal unit.
On the other hand, a GW type terminal enables a real-time communication between a T.38 terminal on the network and a conventional facsimile apparatus, such as a G3 facsimile apparatus, on the PSTN. This is realized by converting a T.30 signal extracted from a packet received from a terminal (IAF or GW) into a modem signal, and transmitting the modem signal to the conventional facsimile apparatus as final destination via the PSTN, and by packetizing a modem signal received from the conventional facsimile apparatus through the PSTN into a T.30 signal, and transmitting to the other terminal via the network.
As described above, a real-time communication between a T.38 terminal on an IP network and a conventional G3 facsimile apparatus on the PSTN is realized by the T.38 compliant GW type terminal converting a T.30 signal received as an IP packet into a modem signal, keeping information content as it is, and transmitting the modem signal to the PSTN, and by converting a T.30 signal received from the PSTN as a modem signal into an IP packet signal, keeping information content as it is, and transmitting the IP packet to the packet network. This means that a transmission speed of the high-speed modem for facsimile message transmission is set up by a T.30-based protocol between the conventional G3 facsimile apparatus connected to the GW type terminal through the PSTN, and an IAF terminal connected to the GW type terminal via the packet network, or another conventional G3 facsimile terminal connected through the PSTN to another GW type terminal that is connected to the packet network.
Specifically, an apparatus on the receiving side provides its capability, including information about a capable transmission speed to an apparatus on the transmitting side by a digital identification signal DIS. The transmitting-side apparatus determines a communication condition within capacity limits of the receiving side apparatus and the transmitting-side apparatus, and the determined communication condition is provided to the receiving side apparatus by a digital transmitting instruction signal DCS so that the same condition as the transmitting-side apparatus is set up in the receiving side apparatus.
Therefore, it has been a prerequisite for the real-time network facsimile communication under the T.38 that network bandwidth allowed to the GW type terminal is sufficient to realize a message data transmission at the transmission speed set up between the terminals by the T.30 protocol.
For above reasons, message data cannot be transmitted and received at the set-up transmission speed if the network bandwidth allowed to the GW type terminal is insufficient for transmission of the message data in the transmission speed set up between the terminals by the T.30 protocol, causing a communication error.
In an office where facsimile is not used frequently, dedicating sufficient network bandwidth (about 64 Kbps) to a GW type terminal for facsimile communications reduces bandwidth for other applications, creating inefficiency. Especially, in a packet network using an ISDN router and the like, available bandwidth is narrow, such as 64 and 128 Kbps, making it difficult to dedicate sufficient bandwidth for the facsimile communication.
These problems arise not only in the ITU-T T.38 compliant facsimile communications, but also in real-time network facsimile communications realized by conversion between a T.30 packetized signal on the packet network and a T.30 signal on the PSTN.